Electric high interrupting capacity fuse for low current ratings



F. J. KOZACKA 3,529,270 ELECTRIC HIGH INTERRUPTING CAPACITY FUSE FOR CURR INGS Fi May 8 Sept. 15, 1910 FIG. I

INVENTOR:

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Y. A T. m m m MQA KQW w W mum w R W F United States Patent O 3,529,270 ELECTRIC HIGH INTERRUPTING CAPACITY FUSE FOR LOW CURRENT RATINGS Frederick J. Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed May 13, 1968, Ser. No. 728,413 Int. Cl. H01h 85/06 US. Cl. 337-232 6 Claims ABSTRACT OF THE DISCLOSURE The casing of an electric fuse having a high interrupting capacity and a small current rating is filled with quartz sand and provided with a fusible element in the form of a cadmium wire having ends that are conductively connected by soft solder joints to the terminal caps closing the casing of the fuse. An additional heat sink is operatively related to each of said pair of caps.

BACKGROUND OF THE INVENTION These prior art fuses perform satisfactorily, except that the high resistivity of their fusible element is conducive to relatively high voltage drops and relatively high i -r losses. This is a very undesirable condition in many applications, particularly where the fuses are housed in a common enclosure together with other heat generating equipment.

Many experiments have been conducted to remove the aforementioned drawback of state-of-the-art electric high interrupting capacity fuses for low current ratings such as, for instance, fuses to be used in an electric system having a circuit voltage of 600 volts, or in excess of 600 volts, having a high available short-circuit current and which fuses have a current rating of less than amps, e.g. 8 amps. These experiments consisted in combining various arc-quenching fillers and fusible elements of various metals. The outcome of these experiments was unsatisfactory to date.

SUMMARY OF INVENTION Electric fuses that are intended to have a high inter casing of insulating material and there is an arc-quenching filler of quartz sand inside of the casing. Both ends of the casing are closed by a pair of terminal caps mounted on the axially outer ends of the casing. A fusible normally current-carrying wire of cadmium is arranged inside the casing, submersed in the aforementioned filler of quartz sand and extends from one of the pair of terminal caps to the other of the pair of terminal caps and conductively interconnects the pair of terminal caps. Joints of a soft solder conductively connect each of the ends of said wire of cadmium to one of said pair of terminal caps.

BRIEF DESCRIPTION OF DRAWINGS taken along IIII of FIG. 1; and

FIG. 3 being an exploded view of the parts forming one of the ends of the fuse of FIGS. 1 and 2.

DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION Reference numeral 1 has been applied to indicate a tubular casing of insulating material housing a pulverulent arc-quenching filler in the form of a body 2 of quartz sand. A pair of terminal caps 3 of sheet metal are mounted on the axially outer ends of casing 1 closing the latter. Each terminal cap 3 has an inherent heat absorbing capacity which depends upon the particular metal it is made of, its mass, etc. The axially inner ends 3a of terminal caps 3 are crimped into casing 1 to firmly secure caps 3 to casing 1. A fusible normally current-carrying wire 4 of cadmium is arranged inside of casing 1 in coaxial relation thereto. Cadmium wire 4 is submersed in the body of quartz sand 2 and extends from one terminal cap 3 to the other terminal cap 3, thus conductively interconnecting terminal caps 3. Joints of soft solder conductively connect each of the ends of wire 4 of cadmium to one Of terminal caps 3.

It is generally very difficult, if not impossible, to solder a wire of cadmium to a terminal cap made of thin sheet metal and having a relatively limited heat absorbing capacity. In the instant case solders having a substantially lower melting point than cadium (which is 320.9 deg. C.) are ruled out because they are likely to melt on account of i -r losses generated at the interfaces between the terminal caps and a fuse holder receiving the same. An acceptable solder would be 40/60 tin-lead solder having a melting point of 238 deg. C. In a soldering operation involving a terminal cap of thin sheet metal and 40/60 tinlead solder it is normally impossible to preclude heating of the terminal cap above the melting point of the solder. Because of the closeness of the melting points of cadmium and 40/60 tin-lead solder, it is normally impossible to use such solder for soldering a fusible wire of cadium to a terminal cap of sheet metal. The cadmium wire tends to melt during the soldering operation because the terminal cap tends to exceed the melting temperature of cadmium. I have discovered that a cadmium wire can safely be soldered to a thin sheet metal terminal cap with 40/60 tin-lead solder if the cadmium wire is embedded in a body of quartz sand since the latter operates as a heat sink limiting the temperature rise of the terminal cap during the soldering operation. It is, however, preferable not to rely entirely on the heat absorbing capacity of the quartz sand to preclude fusion of the cadmium wire during the process of assembly of the fuse, but to provide each of the pair of terminal caps of the fuse with an additional heat sink capable of safety limiting the ceiling temperature which each pair of terminal caps may reach during soldering operations. Such an arrangement has been shown in FIGS. l-3 of the accompanying drawing.

Reference character 6 has been applied to indicate a pair of heat sinks of which each is operatively related to one of terminal caps 3, and this increases the heat absorbing capacity thereof above its inherent heat absorbing capacity. Each heat sink 6 is shaped like a rivet and includes a head or rod portion 6a and a shank portion 6b. Each shank portion 6b projects transversely through one of terminal caps 3 at the center thereof. The ends of cadmium wire 4 project through the same center openings or perforations in caps 3 as the shank or rod portions 612 of heat sinks 6. Each terminal cap 3 defines a recess 3b having the shape of a frustum of a cone and the head portions 6a of heat sinks 6 are arranged in recesses 3b. The axially outer ends of the cadmium wire are bent to conform substantially to the shoulder formed between head portions 6a and shank portions 6b of heat sinks 6. Thus there is an immediate physical engagement between heat sinks 6 and the axially outer ends of cadmium wire 4. The joints of soft solder cover the head portions 6a of heat sinks 6 and fill the recesses 3b in caps 3. A washer 7 of asbestos fibers is interposed between each cap 3 and one of the rims of casing 1. It will be apparent from the Every fusible Wire carrying a current sufiiciently high to cause ultimate melting of the wire undergoes a first heating period during which its temperature is raised to the wires fusing point. The lower the fusing point, the lower the temperature to which the surrounding quartz foregoing that heat sinks 6a, 6b are bounded by caps 3, 5 sand tends to be raised during this first heating period. i.e. that the axially outer ends thereof do not project be- The first heating period is followed by a period during yond caps 3. which the temperature of the fusible wire remains un- There are Various reasons which may cause failure changed and the fusible wire is liquefied. The temperature of a high interrupting capacity quartz-sand-filled fuse. 10 rise of the quartz sand filler during this period tends to be One of these possible reasons is the generation of excesrelatively low if the heat of fusion of the wire metal is sive are energy under major fault current conditions. Anrelatively low. If the liquefied wire is not severed by exother possible reason is formation of a fulgurite during the ternal forces, the temperature of the liquified metal inprocess of reducing a major fault current to zero whose creases from its melting point to its vaporizing point. The dielectric strength is insufficient and that breaks down temperature increase of the surrounding quart filler durunder the action of the recovery voltage. Still another ing this second heating period tends to be relatively lim- Possiblfi reason which may cause failure of a gh i t ited if the difference between the boiling point and the rupting capacity quartz-sand-filled fuse is excessive heatmelting point of the metal of which the wire is made is ing of the quartz sand prior to kindling of an arc in the relatively limited. During vaporization of the fusible Wire fuse. This kind of failure may occur at small current its temperature remains unchanged until the liquid curintensities barely exceeding the minimum fusing current. rent path is broken, and an arc is kindled.

High interrupting capacity fuses having a fusible element The good performance at low current levels of quartzwhose resistance and whose i -r losses are relatively sand-filled high interrupting capacity fuses having a fuslarge when carrying the ra e current, r h minimum ible element in form of a cadmium wire may be attributed fusing current, are particularly susceptible to failure on 5 mainly to the relatively low fusing point of cadmium account of excessive heating of the quartz sand prior to (320.9 deg. C.), the relatively low latent heat of fusion kindling of an are inside of the fuse. (l3 cal./gram) of cadmium, and the relatively low differ- Considering a current having an AC voltage whose ence (416.1 deg. C.) between the melting point and the R.M.S. value is E, whose self-inductance is L, who'se boiling point of d i external resistance is R, and that includes a fuse whose Zinc, lead and tin wires have been tried in combinafusible element has a resistance r, the following e-quation with quartz sand fillers and the performance of such tlon obtalns for the R.M.S- va e O e Current I. fuses at low current intensities has been found to be lack- E ing, or unsatisfactory. (1) It will be understood that I have illustrated and de- I scribed here1n a preferred embodiment of my invention, The lncrease 1n mp r of {11511316 Wlre carry and that various alterations may be made in the details g a electric current ep PP Its l e v, the thereof without departing from the spirit and scope of speclfic heat of the rnatenal of whlch the wire 1s made, the invention as defined in the appended claims. the surface 0 of the wire and a coefficlent 5 of heat trans- I 1 Q The followlng equatlon lnterrelates these 40 1. An electric high interrupting capacity fuse for low quantltlesl current ratings including in combination:

l 19 My: pr (a) a tubular casing of insulating material;

dt (b) an arc-quenching filler of quartz sand inside said 2 caslng; F (c) a pair of knife-contactless terminal caps mounted (2) on the axially outer ends of said casing and closing The first term in this equation refers to heat generated 5 each of Sald.pa1r of tqmmal caps havmg inside the fuse and the second term refers to heat dissipated an m F h absorbnig capaclty: from the fuse (d) fus1ble wire of cadmlum arranged lnslde of sa1d The resistance r of the fusible wire depends, on the other Submerged sa1d finer of quartz sand and hand, on its temperature, as is apparent from the followextendmg from one of sa1d P of terminal caps to ing equation the other of said pair of terminal caps and conduc- (1+ 3 tively interconnecting said pair of terminal caps; wherein r is the resistance of the fusible wire at room (e) a D of Jomts of Soft Solder. eacih cOnducuYdy temperature and wherein a is the temperature coeflicient connectmg i the sa1d Wire of cadmlum of resistance. to one of sa1d pa1r of termlnal caps; and

It is apparent from the above that establishing a gen- (f) a P of heat slPks f on one of salfl of eral solution of Equation 2 is a rather complex matter. l l Caps each m adfiltlon to one of sa1d The data included in the following table and their fol- 6O lolnts of soft solder to Increase the heat absorbmg lowing discussion are conductive to an appreciation of p y of each of Said P of terminal p beyond the present invention without an elaborate mathematical Said inherent heat absorbing p y thereof to treatment of the physics thereof. safely limit the ceiling temperature which each of Silver, Copper, Zinc, Lead, Cadmi- Tin, Ag Cu Zn Pb um, Cd Sn Initial resistivity at 20 deg. 0., mi-

crohm centimeters 1.59 1. 771 5. 8 22 7. 6 11. 5 Temperature coefliclent of solid resistance at 20 deg. C 0. 0038 0.00382 0. 0037 0. 0039 0. 0038 0.0042 Melting point, deg. C. 960. 5 1083 414. 4 327. 43 320. 0 231.8 Boiling point, deg. C 1930 2340 907 1613 737 2360 Boiling temperature above melting point, deg. 969.5 1257 492.6 1285.57 446.1 2128.2 Specific heat per on volume at 18 deg. C 0. 056 0. 094 0. 092 0. 030 0. 08 0. 054 Latent heat of fusion per unit volume,

cal. lgram 50 said pair of terminal caps may reach during soldering operations.

2. An electric high interrupting capacity fuse as specified in claim 1 wherein said joints of soft solder are of 40/60 tin-lead solder.

3. An electric high interrupting capacity fuse as specified in claim 1 wherein said heat sink includes a head portion situated on the outer surface of one of said pair of terminal caps and a shank portion projecting transversely through one of said pair of terminal caps at the center thereof, said fusible wire having ends projecting transversely through each of said pair of terminal caps at the center thereof and being bent to conform substantially to the shoulder formed between said head portion and said shank portion of said heat sink, each of said joints of soft solder covering said head portion of said heat sink of one of said pair of terminal caps.

4. An electric high interrupting capacity fuse as specified in claim 3 wherein each of said pair of terminal caps defines a recess having the shape of a frustum of a cone, and wherein said head portion of said heat sink of each of said pair of caps is arranged in said recess thereof.

5. An electric high interrupting capacity fuse for low current ratings including in combination:

(a) a tubular casing of insulating material;

(b) a pair of blade-contactless terminal caps mounted on the axially outer ends of said casing and closing said casing, each of said pair of terminal caps having a recessed portion in the form of a frustum of a cone forming the axially outer end surface thereof and said recessed portion of each of said pair of terminal caps defining a performation in the center thereof;

() a fusible Wire of cadmium arranged inside said casing extending from one of said pair of terminal caps to the other of said pair of terminal caps and conductively interconnecting said pair of terminal caps, said wire of cadmium having axially outer ends each projecting into said recessed portion of one of said pair of terminal caps through said perforation in the center thereof;

(d) an arc-quenching filler of quartz sand inside said casing submersing said Wire of cadmium, said quartz sand being capable of exerting an intense action of heat dissipation away from said wire of cadmium while said Wire of cadmium is still intact;

(e) a pair of heat sinks substantially in the shape of short rods each arranged inside said recessed portion of one of said pair of terminal caps, each of said pair of heat sinks being confined to said recessed portion of one of said pair of terminal caps and each of said pair of heat sinks having an axially inner end projecting through said perforation in the center of said recessed portion of one of said pair of terminal caps and in physical engagement with one of the axially outer ends of said wire of cadmium; and

(f) a pair of joints of soft solder each in said recessed portion of one of said pair of terminal caps and each in physical engagement with one of said pair of terminal caps, one of the axially outer ends of said Wire of cadmium and one of said pair of heat sinks.

6. An electric high interrupting capacity fuse for low current ratings including in combination:

(a) a tubular casing of insulating material;

(b) an arc-quenching filler of quartz sand inside said casing;

(c) a pair of terminal caps mounted on the axially outer ends of said casing, closing said casing and each having an inherent heat absorbing capacity, each of said pair of terminal caps further having a recessed portion in the form of a frustum of a cone forming the axially outer end surface thereof and said recessed portion of each of said pair of terminal caps defining a perforation in the center thereof;

(d) a fusible wire of cadmium arranged inside said casing, submersed in said filler of quartz sand, extending from one of said pair of terminal caps to the other of said pair of terminal caps and conductively interconnecting said pair of terminal caps, said Wire of cadmium having axially outer ends each projecting into said recessed portion of one of said pair of terminal caps through said perforation in the center thereof;

(e) said recessed portion of each of said pair of terminal caps being filled with a mass of 40/60 tin-lead solder conductively connecting each of the ends of said Wire of cadmium to one of said pair of terminal caps; and

(f) a pair of substantially rod-shaped heat sinks each having an axially outer end confined to the space of said recessed portion of one of said pair of terminal caps and each having an axially inner end projecting out of said recessed portion of one of said pair of terminal caps through said perforation in the center thereof, each of said pair of heat sinks being substantially submersed in said mass of 40/60 tin-lead solder in said recessed portion of each of said pair of terminal caps, and each of said pair of heat sinks increasing the heat absorbing capacity of one of said pair of terminal caps above said inherent heat absorbing capacity thereof to safely limit the ceiling temperature which each of said pair terminal caps may reach when said recess in each of said pair of terminal caps is being filled with said 40/ tin-lead solder.

References Cited UNITED STATES PATENTS 3,238,336 3/1966 Kozacka 337159 2,159,423 5/1939 Bussmann 33729O XR 1,208,448 12/1916 Arthur 337-290 GEORGE HARRIS, Primary Examiner D. M. MORGAN, Assistant Examiner US. Cl. X.R. 337-248, 290 

